[1] The Externals are all mutants with the additional gift of immortality and, as expressed by Cable, they play an important role in Apocalypse's rise to power at least in one future.
Saul, Absalom, Gideon, Nicodemus, Burke, Selene, Candra, and Crule (to a lesser extent) formed the self-fashioned "High Lords", bent on playing power games and manipulating events behind the scenes.
[7] Selene ultimately escaped, but not before summarily dismissing the assumption that Cannonball was an External, calling into question Cable's conclusion and the reasons for it.
He summoned his fellow Externals to the Eternal Caldera on Krakoa, revealing his intentions to sacrifice some of their life energy to power the Arak Maw Gate.
With the unexpected help of Rictor, the group wiped out four of the Externals and extracted their life energy to fuel the crystals that powered the gateway to Arakko.
Although she had been working with Sebastian Shaw and Orchis for a time, Selene eventually returned to her base in Nova Roma, where she began using Absalom, Burke, Gideon to gather in mutants from across the global in order to achieve Dominionhood.
When the time-traveling Legion killed his own father and created an alternate reality, it is unknown what happened to the Externals, as only a few had made their presence known as soldiers of Apocalypse.